The goal of this project is to develop a protocol for successful cardiac allografts across major histocompatibility barriers. We intend to adapt a protocol successful in inbred rats to random-bred goats. LEW rats are lethally irradiated and grafted with WF hearts on day O. A mixture of LEW marrow and thymus cells are infused on either the same day or 2 days later. Delaying the repopulation two days results in permanent grafts in 85 percent of the cases. We intend to determine the mechanism of "tolerance" induction in the rat model. BIBLIOGRAPHIC REFERENCES: Rodday P, Bennett M. and Vitale J. 1976. Delayed erythropoiesis in irradiated rats grafted with syngeneic marrow: effects of cytotoxic drugs and iron-deficiency anemia. Blood 48:435-447. Bennett M, Baker E, Eastcott J, Kumar V, and Yonkosky D. 1976 Selective elimination of marrow precursors with the bone seeking isotope 89Sr: Implications for hemopoiesis, lymphopoiesis, viral leukemogenesis and infection. J Retic Soc 20:71-87.